1. Field of the Invention
The present invention relates to an automotive suspension system including a torsion bar for providing roll and/or sway stabilization. The invention further relates to a system for varying the connection location of the torsion bar to a motor vehicle in order to selectively vary suspension characteristics of the vehicle.
2. Description of the Related Art
Torsion bars are used in vehicle suspension systems in various ways, most commonly as anti roll bars for resisting rolling and swaying motions that occur during vehicle movement. In addition, a torsion bar may be used in a vehicle as a spring to serve a weight bearing function. These functions are accomplished by connecting at least one end of the torsion bar to the suspension arm of a vehicle and either fixing the opposite end of the torsion bar to the frame of the vehicle or to another suspension arm of the vehicle.
Torsion bars are generally connected to vehicle suspension arms through a torsion arm extending from the torsion bar. By way of example, and as illustrated in FIG. 1, torsion bar 20 includes integrally formed torsion arms 22 and 24. The torsion bar 20 is rotatably connected to the underside of a vehicle 30 with sleeves 26 and 28. The distal ends of the torsion arms are connected to suspension arms 32 and 34 which in turn support the rear wheels of the vehicle 30.
FIGS. 2a-c, schematically illustrate three ways in which torsion bars may be used in a motor vehicle. In FIG. 2a, torsion bars are used to provide roll and sway resistance. In FIGS. 2b-c, the torsion bars are used to serve a weight bearing function. By way of example, the related art and the present invention will be hereinafter discussed in connection with the arrangement shown in FIG. 2a, which corresponds to FIG. 1. However, the present invention is not limited to this arrangement.
During cornering, the body of a vehicle has a tendency to roll towards the outside of the turn. When this occurs, the suspension arms at the outside of the turn become compressed toward the body of the vehicle while the suspension arms on the inside of the turn move away from the body of the vehicle. A torsion bar connected between the inside and outside suspension arms resists this motion by transferring forces from one side of the suspension system to the other, thereby resisting roll. For example, if the vehicle in FIG. 1 were to turn towards the left at high speed, the suspension arm on the right side of the vehicle would move in an upward direction causing the torsion bar to rotate counterclockwise. This would exert an upward force on the left rear suspension arm countering its natural tendency to move downward. Simultaneously, an upward force would be transmitted to the right side of the vehicle through rotatable mount 26 and a downward force applied through mount 28.
Torsion bars can be constructed of different materials such as spring steel, and in different sizes in order to provide an automobile with desired performance characteristics. For example, provided all other variables remain constant, the higher the torsion modulus of a torsion bar, the greater roll resistance that will be imparted to the automobile. High roll resistance enhances an automobile's steering performance, especially when the vehicle is moving at high speeds and on smooth surfaces. On the other hand, when the vehicle is traveling along rough and uneven surfaces, high roll resistance may decrease traction. In addition, on rough roads, even at slow speeds, a torsion bar with a high torsion modulus provides a bumpy and uncomfortable ride.
Thus, automobile manufacturers are often required to assess the trade-offs between torsion bars of high and low moduli of torsion, and usually have to make a compromise so that handling performance is acceptable under all road conditions. Of course, by making this compromise, handling performance can never be optimized under all road conditions.
In order to overcome this drawback of the prior art, attempts have been made to provide automobiles with torsion bars having selectively variable moduli of elasticity. For example, U.S. Pat. No. 4,796,991, describes a torsion bar split at its midpoint and having an adjuster disposed therebetween. The adjuster includes a chamber filed with magnetic fluid. When a magnetic field is energized, it causes internal friction in the magnetic fluid, thereby increasing the torsion modulus.
In U.S. Pat. No. 4,805,929, a control cylinder having a piston is connected between the wheel suspension of a vehicle and an end of an arm connected to the torsion bar. Fluid is supplied to the cylinder to lock the piston in a fixed position within the cylinder thereby providing stability through the torsion bar, or to permit the piston to move within the cylinder, effectively disengaging the torsion bar.
There are a number of drawbacks with the previously described related art systems. First, they are relatively complex and expensive to manufacture, and are therefore impractical from an economic standpoint. In addition, in the related art systems, if the electrical system of the vehicle fails while the suspension system is in a low stability mode, the suspension system may be prevented from returning to the high stability mode. This could create an extremely hazardous condition which could lead to the vehicle flipping over during a turn.